The present invention is in the field of tire retreading and relates to improvements in the apparatus used for the retreading of tires. In particular, the invention relates to rasp blades, and in an improved assembly of such blades on tire rasp hubs for buffing away the tread on worn tires.
It is becoming increasingly accepted that the retreading of worn tires, also known as casings, contributes positively to both the economic and environmental well being of the community by providing reduced manufacturing and purchase costs and by preventing large numbers of worn tires from polluting and destroying the environment, whether as land fill or discarded otherwise. Also, as tires are petrochemically derived products, large reserves of oil and other energy forms are used in their manufacture, and the resultant polluting emittents are an undesirable by-product. It is generally not well known that most of a worn tire remains useful as the tire body or walls, comprising a significant major portion of the tire, are normally in excellent condition and have about the same strength as a comparable new tire.
The conventional process by which suitable worn tires, or casings, are retreaded is to buff away the worn tread, selectively repair any damage that may remain in the casing after buffing, bond the new tread to the casing by a selected vulcanizing process, cure the rubber so as to harden and shape it into the desired tread design, before final inspection for suitability for use.
In order to buff and cut away the worn tread, the casing is mounted on a buffing machine (or lathe type machine) and inflated. A tire buffing rasp hub, comprising a hub core having a large number of toothed rasp blades mounted thereon, is then rapidly rotated, say, on a motor driven shaft, and the peripheral surface of the casing is moved against the rotating rasp hub so as to loosen, tear and grind off the excess rubber and roughen it sufficiently so that the buffed surface of the casing can bond the new rubber tread in the vulcanizing process. Each tire has a predetermined crown width, profile and radius and the casing must be buffed to the particular shape, size and texture to receive a new tread that ensures optimal tread to road contact. Buffing of the worn tread is therefore a critically important stage of the retreading process.
Rasp blades of the prior art comprise numerous configurations and shapes, a particularly preferred type comprises teeth of an essentially dove tail shape projecting from the outer working edge of an arcuately shaped rasp blade, each of the teeth having a notch cut out from the most distant surface from the body of the blade to form a series of substantially xe2x80x98Yxe2x80x99 shaped teeth separated by recesses of partly incomplete circular shape. The notch formed in each tooth divides it into halves, and each half is offset to opposite sides of the general plane in which the blade lies, thus creating a second edge immediately behind the cutting edge of each tooth. As the rasp hub is rotated, the first or leading edge buffs or cuts the surface of the casing to a texture which markedly improves the bonding of new rubber thereto.
A typical tire buffing rasp hub which includes such rasp blades as aforementioned has the form of a hub core defined by interconnected front and back cylindrical end plates having mounted therebetween arcuate or quadrant shaped rasp blades stacked in four (or more) separate parallel arrays with a partly spiral inclination around the circumferential perimeter of the rasp core. Each rasp blade of any one stack is separated from its adjacent other blade of the stack by spacers, the stack being secured in position between the end plates by support pins through a hole or holes in each rasp blade. Removable fastening means hold the end plates together, sandwiching the rasp blades, thus allowing for dismantling of the rasp hub for purposes of maintenance and repair, such as when the teeth become worn or broken.
In the prior art, all blades of whatever configuration and shape used on tire rasp hubs were of identical shape, size and distribution of teeth for that particular tire rasp hub. For instance, any two adjacent blades in a stack had identical secondary and tertiary configuration and any one tooth of one blade was aligned substantially horizontally (or vertically depending on the set up of the buffing machine) with a tooth of the adjacent blade. This was usually facilitated by fixing each blade of a stack in a xe2x80x9cname downxe2x80x9d direction, whereby the manufacturer""s name which appeared on only one of the two opposed faces of the blade indicated the direction to which the name side of all blades of that stack were to face. Where this technique was not employed, some other means of facilitating the stacking of the blades in a commonly aligned direction was used. Furthermore, all of the teeth on each blade were symmetrically disposed across the working edge of the blade. For instance, the tooth closest to one end was located the same distance from that end as the tooth closest to the other end was located from that other end.
It was found by the present inventor that by off-setting or indexing all of the teeth to one side of their position in prior art symmetrically disposed blades by a quarter of the pitch that separates each tooth in the blade, and by fixing each blade of a stack in a repeating xe2x80x9cname upxe2x80x9d, xe2x80x9cname downxe2x80x9d direction, a superior and longer lasting buffing performance and buffed texture is achieved. The assembly of blades so formed presents one form of a staggered teeth array across each stack of the rasp hub. This arrangement was disclosed in Australian Petty Patent 719,185.
A hub is rotated at high speed anywhere from 1440 to 4,100 rpm or higher. The tire to be treated is also rotated but in an opposite sense to that of the hub. Two types of action are involved in removing the tread. The hub is advanced radially into the tread in a feed pattern and across the tread in a traverse pattern. The wear on blade teeth differs for each of these modes. In a feed pattern, wear is greatest in the middle rows of a hub while in a traverse pattern wear is more evident in the outer rows of the hub. In either pattern wear on a blade in a given row depends on the arrangement of the blades in the row. Where the angle of a blade with respect to the axis and direction of rotation of the hub is positive the blade is referred to as leading and most of the wear is borne by such blades. Moreover, most wear on an advancing blade occurs towards that end of the blade periphery first contacting the tire tread. Blades at a negative angle are called retreating or trailing blades and receive little effective wear. Reversing the rotation of the hub and/or tire changes trailing blades into advancing blades and wear over a hub""s circumference can be made more uniform. This complicates the design and control of the buffing apparatus. In addition, the uneven wear on a blade tends to overheat that section of the blade affecting the quality of the buffing action by the blade by heating the tire rubber.
Thus it is found that a blade tends to wear over only a partial extent of the teeth. This is true also for other forms of prior art rasp blades. For example, in the prior art 4, 5 or 6 blades can circumscribe the hub axis. The wear of the teeth in a 4 bladed arrangement where the teeth are alternatively angled in a leading/trailing sequence only provides a 25% efficiency, that is, only a quarter of the teeth are buffing the tire tread. Obviously, wear dictates when the blade has to be replaced. In addition, friction between the blade teeth and the tire tread heats a blade in this area of disproportionate wear adding another factor to a reduced life for the blade than would otherwise be suggested. Thus considerable wastage occurs with this prior art arrangement.
Also, in the prior art one or two pins support each blade in a hub arrangement. The hub may rotate at a speed of 4,100 rpm or more and the centrifugal forces generated can result in bending, flexing and eventual breakage of a pin. If a blade is supported by a single pin, its breakage may fling the blade from the assembly and unbalance the hub. If a blade is supported by two pins and one breaks, the blade is retained by the second pin but the hub can still become unbalanced. This may cause damage to the hub, the tire or, if a blade is flung off, it may injure persons or property nearby.
The present invention seeks to provide a further improvement in the use of rasp blades and in particular the arrangement as described in Australian Petty Patent 719,185, by increasing the contact region of each rasp blade thereby providing more effective use of a blade and a more efficient buffing action.
It is therefore an object of the present invention to provide a rasp blade and hub assembly that can be used more efficiently.
It is another object of the present invention to provide an arrangement of blades on a tire rasp hub adapted to buff away worn tread from a casing at a rapid rate while developing a minimum amount of heat that might otherwise adversely affect the texture of the buffed surface needed for suitable retreading.
According to a first aspect of the present invention there is provided a rasp blade for buffing tire treads including an arcuate substantially planar base, a row of teeth along a working peripheral edge of said base and means for fixing said base into a hub assembly, wherein said arcuate base extends substantially not more than 45xc2x0 and said row of teeth are in a substantially circular path.
According to a second aspect of the present invention there is provided a rasp blade for buffing tire treads including an arcuate substantially planar base, a row of teeth extending along a working peripheral edge from one end to another end of said base and means for fixing said base into a hub assembly, wherein said arcuate base extends substantially not more than 45xc2x0 and said row of teeth extends along an arc having a variable radius from said one end thereof to said other end thereof, said radius being measured from the rotational axis of said hub assembly and said variable radius being greater at said ends and decreasing towards the center of the blade.
Preferably, said arcuate base and said row of teeth extend along an arc of between 30 and 45 degrees and said means for fixing are a plurality of apertures in said base for accommodating pins of said hub assembly therethrough.
A blade, according to the invention, which extends over an arc of 45xc2x0 has a performance improvement of 50% over a similar prior art blade.
According to a third aspect of the present invention there is provided a rasp blade for buffing tire treads including an arcuate substantially planar base, a row of teeth along a working peripheral edge of said base and means for fixing said base into a hub assembly, wherein said row of teeth extends along an arc having a variable radius from one end thereof to the other, said radius being measured from the rotational axis of said hub assembly.
Preferably, said variable radius is larger at said ends and decreases towards the center of the blade.
According to a fourth aspect of the present invention there is provided a rasp blade for buffing tire treads including an arcuate substantially planar base, a row of teeth extending along a working peripheral edge from one end to another end of said base and means for fixing said base into a hub assembly, wherein said row of teeth extends along an arc having a variable radius from said one end thereof to said other end thereof, said radius being measured from the rotational axis of said hub assembly with which the rasp blade is used, said variable radius being greater at said ends and decreasing towards the center of the blade.
Preferably, said arcuate base and said row of teeth extend along an arc of between 30 and 90 degrees and said means for fixing are a plurality of apertures in said base for accommodating pins of said hub assembly therethrough.
The blades according to the invention are arranged into stacked rows about a hub Preferably, adjacent blades in a row are arranged in parallel or alternate leading/trailing orientations that is, the blades set alternately at a positive or negative angle to the axis of the hub, and/or with teeth in adjacent rows overlapping as described in Australian Petty Patent 719,185.
Preferably, each blade has three apertures for supporting or fixing the blade by pins into a hub arrangement or assembly.